I'll add a few things here...

Current-mode signals are generally less susceptible to electronic noise (=
than=20
voltage-mode signals), so great for sensors in industrial or other noisy=20
environments.

When read by an A/D, the dynamic range (voltage) can be set simply with a=
 load=20
resistor.  (Though I recently had a situation, where to get the voltage-r=
ange=20
I needed, the load resistor would've surpassed the A/D max source impedan=
ce=20
value).  With a simple buffer op-amp with gain=3D1, this issue is solved.

Also, although you lose a bit of range by eliminating the 0-4mA range fro=
m the=20
transducer, it's easy to detect sensor/signal failure (at least in case o=
f an=20
open circuit) or non-existence if the output drops to 0mA.

Yes, there are digital signal outputs that do a lot of this better, but t=
he=20
analog signals are still in wide use today, and generally lower-cost.

Cheers,
-Neil.



On Friday 04 July 2003 06:57, Mike Harrison scribbled:
> Typical 4-20ma transmitters (transducers) are 2-wire devices, powered b=
y
> the line, and regulate the current through them according to the value =
they
> want to transmit.
>
> A typical system setup is a 24v PSU, connected to a transmitter and an
> indicator, all in series.
>
> You take your power from whatever voltage appears across your device, w=
hich
> should be kept as low as practical - ideally below about 10V.
> People like Lin.Tech and Maxim have many 4-20ma examples in app notes,
> which should give you a general idea.
>
> If you need more power than you can draw off the line, you will usually
> need to isolate the 4-20 loop from your  extra power supply.

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